FWIW, bike was my only mode of transport for years before selling out and becoming a cager, so i went through a lot of backpacks over the years.
The one i settled on as being the best (you have to pay for it as you know, but it's the best IMO), is the Kriega R30. Proper rain proof without messing with those roll-out covers others use to call their bag "waterproof".
They do a Triumph badged one, so you're obliged to buy it
Probably designed for motorcycle use, has built in back padding/protection and ergonomic shoulder straps to go with armoured jackets etc.